Composite output quality


28 posts   Page 1 of 2   1, 2
by excollier » Thu Jan 10, 2013 9:33 am
For the first time yesterday I tried out the composite video output to two different small lcd tvs that I have.
I have to say that the quality is very poor, it's like watching underwater ( well not quite) but it is bad, how is anyone supposed to use this with anything other than an expensive hdmi tv? Makes the Pi an expensive cheap computer.
Is there a way to "up" the output resolution via composite?
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by rurwin » Thu Jan 10, 2013 9:59 am
The maximum resolution of a BBC Micro was 640x256, and it was unusable on a TV; anything more than 40 characters wide (320 pixels) was too high a resolution. (The VIC-20 used a 20 character-wide screen resolution.)

It's a tribute the TV technology that the RaspPi composite output works at all.

But what is the native resolution of those LCD TVs? If they only have 320x256 pixels, they are not going to be able to display 656x416 or whatever the effective resolution of the RaspPi's composite output is.
User avatar
Moderator
Moderator
Posts: 2888
Joined: Mon Jan 09, 2012 3:16 pm
by excollier » Thu Jan 10, 2013 10:03 am
Not sure what the tv resolution is, I've had them a few years, I have lost the manuals by now. Even on my Sony Bravia, the composite picture is woeful.
As an aside, can the Pi output HDMI and composite simultaneously, or does it only do one at a time?
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by poglad » Thu Jan 10, 2013 10:06 am
You should be able to configure the framebuffer dimensions to match the physical resolution of your LCD displays, which will avoid any poor scaling. The composite output works fine on my analogue CRT television. Some of the finer details are a bit flickery, because of the interlacing, but watching videos full screen is no problem at all.
User avatar
Posts: 102
Joined: Tue Jul 31, 2012 8:47 am
Location: Aberdeen, Scotland
by poglad » Thu Jan 10, 2013 10:07 am
I've always loved that ATV logo by the way! :D
User avatar
Posts: 102
Joined: Tue Jul 31, 2012 8:47 am
Location: Aberdeen, Scotland
by excollier » Thu Jan 10, 2013 10:11 am
Being born and reared in Staffordshire, ATV was at it's peak when I was growing up.
How do I configure the framebuffer settings, caveat, you are speaking to a relative linux newbie, with failing braincells ( too many get-offs from my motorbikes over the years )
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by poglad » Thu Jan 10, 2013 10:17 am
Using config.txt in your /boot partition, see http://elinux.org/RPi_config.txt#Video_mode_options

If you scroll down, you should fine frambuffer width and height settings. Sorry I can't be more specific, am at work right now! :)
User avatar
Posts: 102
Joined: Tue Jul 31, 2012 8:47 am
Location: Aberdeen, Scotland
by excollier » Thu Jan 10, 2013 11:11 am
Thanks, I will take a look.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by pluggy » Thu Jan 10, 2013 11:22 am
excollier wrote:Being born and reared in Staffordshire, ATV was at it's peak when I was growing up.
How do I configure the framebuffer settings, caveat, you are speaking to a relative linux newbie, with failing braincells ( too many get-offs from my motorbikes over the years )


With a handle like that, I'd wager you're one of Maggie's victims in the eighties.....
Don't judge Linux by the Pi.......
User avatar
Posts: 2077
Joined: Thu May 31, 2012 3:52 pm
Location: Barnoldswick, Lancashire,UK
by excollier » Thu Jan 10, 2013 11:26 am
Spot on.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by excollier » Thu Jan 10, 2013 11:27 am
Spot on. A victim of her policy, lasted until 1992.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by pluggy » Thu Jan 10, 2013 11:32 am
Not much help on composite I'm afraid, never bothered with it. To the extent I butchered the connection off one of my Pi's so it would fit in a box I had.

It was a fair guess since you're from Staffordshire, it was a big mining area.
Don't judge Linux by the Pi.......
User avatar
Posts: 2077
Joined: Thu May 31, 2012 3:52 pm
Location: Barnoldswick, Lancashire,UK
by redhawk » Thu Jan 10, 2013 11:49 am
small lcd tvs


Casio TVs??

Composite video is crap no matter how you look at it the fact you mix chroma luma signals (Y/C) down one cable and then have to split it back into Y and C results in nasty side effects.
Ideally it would have been better if the Raspberry Pi provided an S-Video output but the Broadcom SoC doesn't support this kind of video output only composite.
Anyway the nasty artifacts can usually be well hidden providing your video capture device handles both odd and even fields at the same time like a proper TV.
If your capture device discards certain lines or only captures one field (like some crappy USB video grabbers) you basically you end up with rubbish video quality.

Richard S.
User avatar
Posts: 2923
Joined: Sun Mar 04, 2012 2:13 pm
Location: ::1
by pluggy » Thu Jan 10, 2013 12:00 pm
Composite is easy to 'produce' in software from a purely digital starting point, its an ancient system that looks a bit out of place in this day and age. It's only its cost (next to nothing) that keeps it going. Back in the day (early eighties) having a composite input monitor was a huge improvement over a UHF modulator system used on most home computers of the time (You could read an 80 column text display), but things have moved on a bit in the thirty years since.
Don't judge Linux by the Pi.......
User avatar
Posts: 2077
Joined: Thu May 31, 2012 3:52 pm
Location: Barnoldswick, Lancashire,UK
by redhawk » Thu Jan 10, 2013 12:25 pm
Producing composite video is no more difficult than S-Video in fact many of the old computers and game consoles were already capable of Y/C and could be modded for it. :)
I guess with regards to Broadcom the probably didn't bother supporting Y/C because they expected everyone to be using HDMI instead.

In case anyone was wondering about composite video artifacts here are a few I made from my Pi:

Single field odd or even (commonly used by cheap and nasty USB video grabbers)
Image

Fields mixed / blended
Image

Interlaced video
Image

Interlaced video software enhanced
Image

Richard S.
User avatar
Posts: 2923
Joined: Sun Mar 04, 2012 2:13 pm
Location: ::1
by excollier » Thu Jan 10, 2013 12:45 pm
Mine looks like #1 on both tv sets. Awful.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by cosmo » Thu Jan 10, 2013 1:51 pm
The composite quality depends on the TV/monitor you are using. I get similar results with one of my TVs (and an identical model TV I tried in my office break room)- looks like the first image. Both are a small 19" Panasonic CRT.

My other TV looks much better, also a CRT, a larger 32" Sony. Haven't tried it on my LCD screen as of yet, but that one has an HDMI input, so I wouldn't need to use the composite input (not sure if that TV even has one)
User avatar
Posts: 48
Joined: Mon Nov 19, 2012 8:36 pm
by redhawk » Thu Jan 10, 2013 2:17 pm
I think the problem with LCD TVs they don't actually take the video signal and interlace this like a regular CRT TV.
So either they're showing odd field and then even field (progressive scan) or they're mixing both fields together into one frame.
As you can see from the screen shots interlaced video has considerably less colour artifacts than blended fields.

Richard S.
User avatar
Posts: 2923
Joined: Sun Mar 04, 2012 2:13 pm
Location: ::1
by kalehrl » Thu Jan 10, 2013 3:07 pm
I think you didn't set the output to PAL.
By default, it is set to American NTSC and it gets converted to PAL by your TV.
The conversion produces such artifacts which I also got until I uncommented:
Code: Select all
sdtv_mode=2

in /boot/config.txt.
The picture is much better now and I can't find any difference in picture quality between raspi and my dvd player and satellite receiver.
Posts: 342
Joined: Tue Jul 24, 2012 10:49 am
by excollier » Thu Jan 10, 2013 3:23 pm
Thanks, and pardon my ignorance, but when I have uncommented that line, if I find it, how do I save it before exiting? Cant figure that bit out.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by rurwin » Thu Jan 10, 2013 3:33 pm
You edit it as superuser, because otherwise you wont have permission to write it.

So
pi@raspberryPi ~/ $ sudo nano /boot/config.txt

Make the change and then CTRL-X to exit. (That's hold down Ctrl key and hit the X key.) It will ask if you want to save your changes, hit Y. Then it will ask where to save them, hit Enter.
User avatar
Moderator
Moderator
Posts: 2888
Joined: Mon Jan 09, 2012 3:16 pm
by excollier » Thu Jan 10, 2013 3:38 pm
Thanks, I'll try it tomorrow when I have time.
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by excollier » Thu Jan 10, 2013 4:43 pm
Ok, I'm using PiBang and that line is not in /boot/config.txt. Do I just add it at the bottom of the list? Or at the top,I have no idea.
Here is the entire contents of /boot/config.txt on my Pi with PiBang

core_freq=250
sdram_freq=400
over_voltage=0
disable_overscan=1
arm_freq=800
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am
by Burngate » Thu Jan 10, 2013 4:57 pm
Apparently it doesn't matter

If you're going to add more things, it's probably good to keep similar things together, and so I would move the line
Code: Select all
disable_overscan=1
to the bottom, and add
Code: Select all
sdtv_mode=2
below or above that - that keeps the frequency stuff together, with the video stuff below
User avatar
Posts: 2326
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK
by excollier » Thu Jan 10, 2013 5:05 pm
Thank you, I will try it now to see what happens
User avatar
Posts: 192
Joined: Wed Sep 12, 2012 8:17 am